1. Field of the Invention
The present general inventive concept relates to an image forming apparatus. More particularly, the present general inventive concept relates to a fixing device that fixes a toner image onto a printing medium, and an image forming apparatus having the fixing device.
2. Description of the Related Art
Image forming apparatuses, such as printers, copiers, scanners, multi-function machines, or the like, include fixing devices which fix developer images, such as toner images, which are transferred onto sheets of paper by transferring devices known to those skilled in the art.
In order to meet the recent demand for high-speed image forming apparatuses, a rapid temperature increase of nip zones is required so that fixing devices can reach the fixing temperature as rapidly as possible. Additionally, thermal stability is required so that fixing can be performed while maintaining a constant temperature, which is not affected by thermal disturbances, such as a change in the type of paper supplied to the image forming apparatus.
FIGS. 1A to 1C are sectional views schematically illustrating conventional fixing devices. FIG. 1A illustrates a roller-type fixing device, FIG. 1B illustrates a belt-type fixing device, and FIG. 1C illustrates another belt-type fixing device including a nip forming member.
In FIG. 1A, the conventional roller-type fixing device includes a pressing roller 10 and a heating roller 20 which rotate while tightly in contact with each other, and a heat source 30 which is mounted in the heating roller 20.
The conventional roller-type fixing device configured as described above applies heat and pressure onto a non-fixed toner image transferred onto a surface of a sheet of paper P by a transferring device known to those skilled in the art, while the sheet of paper P passes through a nip N formed by pressure contact between the pressing roller 10 and the heating roller 20 which rotate in contact with each other, and then fuses the toner image onto the sheet of paper P. The conventional roller-type fixing device is beneficial in high-speed printing because a temperature decrease when feeding sheets of paper is relatively small. However, since the heating roller 20 has a large thermal capacity, it is difficult to rapidly raise the temperature. The nip N is formed using a pair of rollers 10 and 20, and thus there are limitations to how stably a nip width can be maintained.
Referring to FIG. 1B, another conventional belt-type fixing device includes a pair of pressing rollers 11 and 12 to form a nip N, a fixing belt 22, a tension roller 40 to elastically support one side of the fixing belt 22, and a heat source 31 to apply heat to the fixing belt 22.
The conventional belt-type fixing device configured as described above directly applies heat to the fixing belt 22 using the heat source 31, such as a heat lamp, at a region other than the nip N, and accordingly the fixing belt 22 may have a small thermal capacity. Therefore, the conventional belt-type fixing device is beneficial in a rapid temperature increase. However, a tensile force is applied to rotate the fixing belt 22 which wears out the fixing belt 22. Additionally, the temperature of the fixing belt 22 itself may rise rapidly, but actually the temperature of the nip N may rise at a relatively low rate due to the high level of heat loss which is caused by the fixing belt 22 rotating in contact with the pair of pressing rollers 11 and 12 and with the tension roller 40. In addition, the nip N is formed using the pair of pressing rollers 11 and 12, and thus there are limitations to obtain a stable nip width.
Referring to FIG. 1C, another conventional belt-type fixing device including a nip forming member includes a pressing roller 10, a fixing belt 22 to rotate by a rotation force transferred from the pressing roller 10, a nip forming member 50 to form a nip N at a contact area between the pressing roller 10 and the fixing belt 22, a tension roller 41 to elastically support one side of the fixing belt 22, and a heat source 32 which is mounted in the tension roller 41.
The conventional belt-type fixing device including the nip forming member configured as described above in FIG. 1C enables a width of a nip N to increase, because the nip N is formed by applying pressure to the nip forming member 50. However, when both ends of the nip forming member 50 are pressed against the pressing roller 10 to form the nip N, the nip forming member 50 may be bent, so it is difficult to ensure a stable nip width across the sheet of paper P. Accordingly, edge portions of the sheet of paper P are in complete contact with the pressing roller 10, so that a sufficient nip width can be obtained. However, a central portion of the sheet of paper P is not completely in contact with the pressing roller 10 due to skew of the nip forming member 50, so it is difficult to obtain a sufficient nip width, which thereby causes inferior fixation.